1. Field of the Invention
The present invention is related to the field of steam turbine engines and, more particularly, to a system for maintaining the elevated temperature of vapor transiting the turbine path.
2. Description of the Related Art
Typically, in steam turbine systems, as steam expands isentropically across the low pressure range of the turbine cycle prior to reaching its intended exhaust pressure, its thermodynamic state conditions reach a point of crossing the saturation curve of the vapor. The result is the development of "wet" vapor conditions in the expanding steam turbine medium. A "wet" vapor condition is that condition resulting when droplets of liquid phase turbine medium form in the vapor mass undergoing expansion to progressively lower temperature and pressure. The presence of these moisture particles in the vapor stream moving through the turbine impacts the back surfaces of the blading in the turbine, causing both a reduction in the energy delivered to the blading for the purpose of rotating the turbine shaft, i.e., a reduced energy output, and erosion of the blading edges and pitting of the blade surfaces. The combined effects result in both reduced thermodynamic efficiency of the turbine and increased cost of maintenance of the turbine in service.
Historically, efforts to reduce or minimize these unwanted effects have led to development of what has become known as the "reheat" cycle. When the vapor is developed using a boiler, the boiler delivering heat through consumption of one or another type of fuel, it commonly happens that some of the heat generated by consuming the fuel is not completely absorbed by the vaporizing turbine medium. Having delivered the peak temperature available to the water being boiled, additional heat content at a lower temperature remains available in the boiler passages. After a portion of the higher temperature stages of the turbine path have been traversed in the turbine, the expanding vapor is removed from the turbine, returned to the boiler, and reheated by that source of additional external heat remaining in the boiler. The boiler section providing that lower temperature heat for subsequent addition to the expanding vapor is commonly referred to as the boiler "economizer". In the process, the vapor acquires a higher temperature, at its now reduced pressure, to recreate a superheated condition at its new combination of pressure and temperature. Reintroduced to the turbine path, the reheated vapor moves the expansion path away from the saturation curve, thereby eliminating formation of wet vapor conditions for an ensuing portion of the expansion path.
With steam as the thermodynamic medium expanding in the turbine, eventually a limitation is reached on the ability to expand the steam further due to inability to maintain a high enough vacuum condition to provide the lowest exhaust pressure consonant with the lowest ambient temperature to effect condensation of the exhaust. Current steam turbine practice is limited to a minimum exhaust pressure of about 1.5 ins. Hgabs (3.81 cm Hgabs). This limit is created not by virtue of an inability to secure a lower ambient temperature to effect condensation of the exhaust, but by an inability to maintain that high a vacuum in the condenser. At that very low exhaust pressure, the saturation temperature remains slightly above 91.degree. F. (32.780.degree. C.). Despite the existence of external ambient site cooling conditions being commonly available at much lower temperatures than that, it often happens in American practice that utility company power plants are forced to raise the exhaust pressure in the summer time by an amount sufficient to result in a differing "summer rating" and "winter rating" for their reliable power delivery capacity. Occurrence of a lower temperature in the exhaust conditions as minimum pressure is reached further exacerbates the probability of occurrence of excessive wet vapor conditions before the intended turbine exhaust pressure is attained.
In many applications, where access to the external heat source vaporizing the medium does not permit returning the vapor to its external heat source, the opportunity to institute a reheat cycle may not be available. Damaging moisture content is experienced at higher pressures, or acceptance of higher exhaust pressures with attendant lower thermodynamic efficiencies is forced upon the operator.
Moisture developing in the expansion path is particularly common in power plant facilities operating in geothermal power developments where the geothermal resource being tapped is from a liquid dominated reservoir. The very process of flashing the liquid to yield a vapor fraction produces both the vapor desired to be supplied to the turbine, and a quantity of hot liquid phase brine accompanying the vapor, both at saturation conditions for the temperature and pressure at which the flash occurs. The characteristic saturation curve of steam indicates that after starting from a saturated condition at the turbine admission port, any amount of expansion along an isentropic path through the turbine immediately crosses the saturation curve and enters a wet vapor condition, and becomes progressively wetter as expansion proceeds.
Additionally, at the same time that the steam fraction is released as a vapor, the residual liquid geothermal brine, also at saturation temperature and pressure, becomes a waste stream for the facility that needs to be disposed of by re-injection in the well field. In general, most geothermal brines contain dissolved minerals and other pollutants which impose a minimum temperature at which the brine liquid residue must be re-injected to avoid releasing the dissolved pollutants from the fluid and causing damaging deposits as the solutes separate from the liquid. As an example, it may be necessary to reinject the brine at a temperature of not less than 180.degree. F. (82.2.degree. C.). With the geothermal brine at a starting temperature of 304.degree. F., the prior art process obligated the operation to waste some two-thirds of the heat energy contained in the brine delivered to the surface plant from the well field.